1. Field of the Invention
The invention relates to the area of power control in wireless systems and more particularly the present invention relates to a method and system for providing closed loop power control in narrow band or spread spectrum wireless systems.
2. Description of the Related Art
In the field of wireless communications, several technologies exist for controlling communications between a mobile station, such as a cellular telephone or personal communication system (PCS) handset, and a wireless base station. In a narrow band wireless network, time and frequency slots are partitioned in order to avoid material interference between users. Users can share the same time or frequency slot provided they are sufficiently far apart and such users are known as co-channel users. The issue of power control arises because the co-channel users interfere with each other. Carrier/Interference (C/I) balancing schemes have been described in which C/I is balanced to provide a distribution of the interference such that all users have the same C/I or have the same carrier to interference ratio (CIR), i.e. the ratio of the power level of a desired signal received at a given location to the power level of all other received signals of the given location, or signal-to-interference ratio SIR, see Zander, “Performance of optimum transmitter power control in cellular radio systems,” IEEE Trans. Veh. Technol., Vol. 41, pp. 57-62, February 1992; and, “Distributed Cochannel Interference Control in Cellular Radio Systems,” IEEE Trans. Veh. Technol., Vol. 41, pp. 305-311, August 1992; and Grandhi et al., “Centralized Power Control In Cellular Radio Systems,” IEEE Trans. Veh. Technol., Vol. 42, No. 4, pp. 466-468, November 1993; and, “Distributed Power Control In Cellular Radio Systems,” IEEE Trans. Comm., Vol. 42, No. 2/3/4, pp. 226-228, February/March/April 1994.
In code division multiple access (CDMA) systems, users share the same frequency all the time by using a specific spread spectrum pseudonoise code for each user. Fundamental to the CDMA system is power control. The CDMA system is an interference limited system in the sense that the system capacity, related to the number of simultaneous calls, is a function of the maximum amount of interference that the system can tolerate. In order to maximize the system capacity, the transmitted power of each mobile unit is controlled so that its signal arrives at the cell site with the minimum allowable SIR. Power control is used to mitigate the “near/far problem” preventing users that are geographically closer to the base station from “over-powering” users that are farther away.
Open loop and closed loop power control schemes have been described. The goal of open loop power control is to adjust the transmitted power according to changes in the received power. In the open loop power control method according to IS-95, the mobile station uses the measured total received power along with typical values of certain base station parameters to get a rough estimate of the transmission loss between the unit and the base station. Based on these measurements, the forward link transmission loss is estimated and used to determine the proper open loop power control setting for the mobile station transmitter. The mobile station's transmit power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level. All mobile stations use the same process, and ideally their signal will arrive with equal power at the base station. See “Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System.”
U.S. Pat. No. 6,101,179 describes a method for open loop power control in a CDMA communication system including calculating in the base station a base station pilot transmit power and a base station received sensitivity value. The base station transmits the pilot transmit power value and the receiver sensitivity value to the mobile base station. The mobile station calculates a mean output power in response to the base station pilot transmit power value and the base station receiver sensitivity value. The open loop control can cope only with very slow shadow fading.
In closed loop power control, the base station measures the relative received power level of each associated mobile station and compares it to an adjustable threshold. A determination is made to transmit a power up command or a power down command to the mobile station. The mobile station can make received adjustment commands with open loop estimates to obtain the final value for transmitted power. The goal of the closed loop power control is to provide rapid corrections to the open loop estimate in order to obtain the optimum transmit power.
A Quality-of-Service (QoS) based closed-loop power control performs better than the power-level based approach. The quantity used to measure QoS is the SIR, or Eb/I0. The IS-95 power control system is an up/down hard decision type. If the actual SIR is lower than an SIR target value, the transmission power is raised by a fixed step size, such as 1.0 dB, or 0.5 dB. Alternatively, if the actual SIR is higher than an SIR target value, the transmission power is reduced by a fixed step size, set as 1.0 dB, 0.5 dB. A conventional power control scheme is called the Distributed Constrained Power Control (DCPC), and is given by S Grandhi, J. Zander and R. Yates, “Constrained Power Control,” Wireless Personal Communications, Vol. 1, pp. 257-270, 1995 as:                                           p            i                    ⁡                      (                          k              +              1                        )                          =                  min          ⁢                      {                                                                                γ                    i                    tar                                                                              γ                      i                                        ⁡                                          (                      k                      )                                                                      ⁢                                                      p                    i                                    ⁡                                      (                    k                    )                                                              ,                              p                max                                      }                                              (        1        )            
U.S. Pat. No. 6,070,086 describes a method for closed loop power control. A closed loop power control unit is coupled to respective cell-site transmitter/receiver comprising: means for measuring Eblo (the ratio of signal energy per bit to the interference power spectral density), means for generating power adjustment commands corresponding to deviation in corresponding cell-site Eblo measurement a predetermined Eblo level; wherein the coherent detection schemes are used for reverse link (mobile to cell) in the cellular mobile telephone system and wherein it is assumed that the mobile station is capable of receiving the power adjustment commands and adjusts the transmission signal power in correspondence to the power adjustment commands.
The control of power levels of signals transmitted from devices to base stations may be either centralized or distributed. In centralized power control techniques, a single controller determines the power level for each device in the cell, and communicates that level to each device. Centralized control is advantageous in that a desired CIR level can be achieved immediately since the centralized controller has information about devices in contact with the base station (e.g. about which devices will terminate or initiate communications in a time interval). Centralized control, however, needs all the information of all the devices (including all the link gain) and involves the added infrastructure of a central control mechanism thereby resulting in added network vulnerability due to the single point of control. Centralized control schemes have been described in J. Zander, “Performance of optimum transmitter power control in cellular radio systems,” IEEE Trans. Veh. Technol., Vol. 41, pp. 57-62, February 1992; and, Grandhi et al., “Centralized power control in cellular radio systems,” IEEE Trans. Veh. Technol., Vol. 42, No. 4, pp. 466-468, November 1993. Distributed control, in contrast, only local information is needed for each device and can be an iterative approach in which power levels are adjusted based on feedback from the devices.
Recent work has therefore emphasized distributed, or local, control. In a distributed power control network, the power level of each device is guided using only local measurements, so that eventually all base stations meet any specified CIR requirements. Such power control methods typically adjust the power levels in communications devices based on a determination of a mean (which is a first order statistic) of the interference level at a base station. See, e.g., J. Zander, “Distributed Cochannel Interference Control in Cellular Radio Systems,” IEEE Trans. Vehic. Tech. Bol. 41(3), pp. 305-311, August 1992; G. J. Foschini and Z. Mijanic, “A Simple Distributed Autonomous Power Control Algorithm and its Convergence,” IEEE Trans. Vehic. Tech, Vol. 42 No. 4), pp. 641-646, November 1993; and S. V. Henly, “Capacity In A Two Cell Special Spectrum Network,” 30th. Annual Conference on Communication, Control and Computing, Allerton House, Monticello, Ill., pp. 426-435, (1992). Distributed control schemes can use only a user's own link gain or CIR/SIR to determine power control, as Gandhi, et al., “Distributed Power Control In Cellular Radio Systems,” IEEE Trans. Comm., Vol. 42, No. 2/3/4, pp. 226-228, February/March/April 1994.
U.S. Pat. No. 5,956,649 describes a method and apparatus which uses a set of parameters characterizing an interference signal at a base unit for determining power levels for signals transmitted from a communication device to the base unit. The set of parameters comprises second or higher order statistics characterizing the interference signal, and the parameters are used to determine a desired power level for signals received at the base unit. The desired power level is communicated to a communications device via a pilot signal transmitted by the base unit at a predetermined level. The predetermined level and the power of the received pilot signal are used to compute a path gain between the base unit and communications device. The path gain and desired power level are then used to determine the power level of signals transmitted from the communications device to the base unit.
It is desirable to provide an improved closed-loop SIR based power control scheme.